Patent GB-1120589-A (GB)
Title: Method of and apparatus for photographic reproduction with tone correction
IPC: H04N1/207
Filed: 1966-02-11
Abstract: 1,120,589. Copying by scanning. CIBA Ltd. 11 Feb., 1966 [12 Feb., 1965], No. 6276/66. Heading H4F. In a process for the photographic reproduction of a photographic negative or positive, electrical signals are derived by scanning the negative or positive with a single light source, the electrical signals being proportional to the density of each image point scanned and being used to modify the light source in accordance with a tone correction programme to provide a predetermined density range on a copying material which is exposed to the same light source as the negative or positive. As shown, Fig. 1, a cathode-ray tube 1 is arranged to scan over an original transparency 3 which is in contact with a sheet 4 of unexposed photographic film to produce an image of the transparency on the film. A first electrical signal is produced by means of a semi-reflecting mirror 5 and a photo-multiplier 7 which is dependent on the intensity of the light beam from the cathode-ray tube as it scans each individual image point. A second electrical signal is produced by a photomultiplier 9 in a light collector 8 which is dependent on the intensity of the light beam as it scans each of the image points reduced by the transmission of each corresponding image point. These two signals are passed via logarithmic amplifiers 10 and 11 to a comparison circuit 12 where they are subtracted and the resultant signal, which is dependent on the density value of each individual image point, is passed via a linear gain controller 13 and a function transmitter 14, which constitutes a programmable computer with non-linear transmission characteristics, to a comparison circuit 15, i.e. a subtraction circuit. The second input to comparison circuit 15 is the second electrical signal after it has passed through the logarithmic amplifier 11 and a second non-linear gain controller 16, and the comparison circuit 15 is arranged to provide a signal for controlling the intensity or scanning velocity of the scanning beam of the cathoderay tube so as to maintain the difference between the two inputs to the comparison circuit 15 substantially zero. The photographic film 4 may be separated from the transparency 3 so as to provide an image at a modified scale, Fig. 9a, not shown, and a second partially silvered mirror (22), Fig. 9b, not shown, may be located between the transparency and the photographic film for reflecting the light on to photo-multiplier 9. The photographic film may be arranged in front of the transparency so as to be scanned directly by the light beam to provide a mask for subsequent use with the transparency to provide multiple copies thereof in known forms of copying apparatus. The velocity of the deflection of the scanning beam may be controlled by a deflecting circuit 41 for the cathode-ray tube, Fig. 10, which is controlled by the output of logarithmic amplifier 10. The output of deflecting circuit 41 is also passed through a logarithmic amplifier 42, which detects the instantaneous deflection velocity and hence the local image point duration, to a subtraction stage 43 where it is subtracted from the signal from logarithmic amplifier 11, the output of subtraction stage 43 being passed to gain controller 16, Fig. 1. The transparency may be split up into a number of image areas by placing a line screen in front of the transparency and in front of the photographic film for half-tone prints, alternatively a first coloured screen may be located in front of the photographic film, a second coloured screen of the same colour being located in front of photo-multiplier 9 for suppressing the modulation of the scanning beam of the first coloured screen. The Specification includes a detailed mathematical analysis of the signal relationships.